The case in Osage County was the first investigation presented to the newly-formed Federal Bureau of Investigation (FBI). It was led by J. Edgar Hoover, whom Leonardo DiCaprio portrayed in J. Edgar (2011).
Martin Scorsese said that when he read David Grann's book "Killers of the Flower Moon," he knew that he had to make it into a movie. Scorsese spent several hours together with Chief Standing Bear to convince the Osage Nation to help with the filming.
Lily Gladstone was in the process of registering for a data analytics course and entering her credit card information when a Gmail notification alerted her to a request for a Zoom meeting with Martin Scorsese. As this was during the height of the COVID-19 pandemic, Gladstone was considering a career change due to a dry spell. According to Leonardo DiCaprio, "There was no reading. Marty just instinctively knew Lily was the one. There was a truthfulness in [her] eyes that he saw even over a computer screen. I've never known [Scorsese] meet somebody and then immediately afterwards have this gravitational pull and instinct to say, 'Let's not wait another minute.'"
The author of the book on which the film is based, David Grann, spent a decade researching the material for his acclaimed book, "Killers of the Flower Moon: Oil, Money, Murder and the Birth of the FBI."
During the scene where Ernest called himself a handsome devil, Mollie chuckles while Ernest looks pleased. According to Martin Scorsese, Leonardo DiCaprio improvised the line and Lily Gladstone's laugh and DiCaprio's reaction were genuine. Scorsese said that was the moment DiCaprio and Gladstone became a unit as actors. DiCaprio and Gladstone later clarified that the two of them had discussed the scene and it was Gladstone who had suggested the line to DiCaprio, though their reaction to the delivery was spontaneous.